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Zu Soul Superfly 8 ohm experiment

Have been Zu owner for many years starting with Tone progressing to Druid -then Essence and now Soul with nano tech drivers. Drive the speakers with Unison Research Preludio which is a 15 watt single ended class a amp which utilizes 6 ohm output taps. After reading a review of the speakers which suggested the speakers sounded best when paired with an amp that utilizes 16 ohm output taps i posed the question to Sean. he suggested i could experiment altering the impedance of the speakers by utilizing a 16 ohm resistor to lower the impedance to 8 ohms. I have generally been happy with the sound of my system except for a bit of hardness/glare in the higher frequencies that shows up on some recordings . i attributed this to the recording technique as ell as less than optimal listening environment (10x12 room ) also wondered if perhaps the preludio was not happy with the 16 ohm impedance. Decided to give Seans' suggestion a try and purchased a pair of 15 ohm Mills resistors from parts Express. Wrapped the leads around each of the speaker terminals -placed the spade lugs of the cables over the leads and tightened down the connector. have to admit I did not expect much, but was pleasantly surprised. Thought the speakers sounded cleaner - vocals clearer and a definite decrease in the high end glare/hardness . After changing speaker cables I had been bothered by what seemed to be an exaggeration of sibilance . but noticed that it now sounded much more natural and the hardness associated with it improved. wonder if any other Zu owners have had a similar experience ?
dcr
213cobra
885 posts
 
Impedance matching between amp outputs and speaker can have strong effects. Resistors are cheap and easy, but if you settle on the advantage, here's an even better way to do it:

http://anticables.com/autoformers#!/~/category/id=3461167&inview=product14644819&offset=0&sort=normal

The AntiCables autoformers for load matching do a superb job and improve dynamics over going the resistor route.

Phil
giantbunnyhead
1 posts
 
I have a pair of Zu Druids MK IV 08 and added a 51 ohm Mills resistor across the terminals of the tweeter. The overall impedance (technically just resistance as measured by my ohm-meter) was 7.9 ohms on each side.

The harshness was definitely removed and sound level was altered given the same volume position on the control amp. The things is that I also replaced the Kimber Kap with a Mundorf silver oil cap, which would have helped soften some of the high end bite. Overall though, it's a more pleasant sound to my ears than before the mods. It's also a pretty easy mod to reverse.
dcr OP
10 posts
 
Funny you should mention the anticable site as a friend talked to me about it as well. Really happy with the sound of the speakers now and will probably leave the resistor in place for awhile. Speakers seem more coherent- bass response/detail is improved and sound stage width and depth greater. Noticed that the Zu Union and new Soul II both utilize an 8 ohm driver. I wish that Zu would offer an 8 ohm driver version of the Superfly driver as there seems to be few amps that are optimized for or have a 16 ohm tap.
atmasphere's avatar
atmasphere
11,841 posts
 
Its really important with a transformer-coupled amplifier to load the tap of the output transformer at the impedance it is designed for.

If you put a 16 ohm load on the 8 ohm tap, the transformer can ring (add distortion) which will make it brighter and harsher. Conversely loading the 8 ohm tap with 4 ohms will load the output tubes too hard and will also cause additional distortion, in addition to a loss of power.

However putting a 16 ohm resistor in parallel with a 16 ohm loudspeaker means that although the result is 8 ohms, half of the amplifier power will be turned into heat in the resistor. If you have plenty of power maybe that is not a problem, although the resistor might get quite warm.

Obviously a more elegant solution is to have an amplifier with a 16 ohm tap. FWIW there are a few amps that are optimal when driving 16 ohms. That would be- any amp with a 16 ohm tap and most OTLs. Transistors, although they make less power into 16 ohms, also make less odd ordered harmonic distortion (IOW they sound smoother). It strikes me as a bit odd that a tube amp, if transformer coupled, would not have a 16 ohm tap.
dcr OP
10 posts
 
Thank you for the information . That explains why the speakers sound cleaner/clearer now as the distortion has been reduced. Checked the resistors for heat and did not seem even warm to the touch after 1.5 hours of listening. I am happy with the amplifier,although with a fixed output it may not be as versatile as i like. Agree that the resistor is not an ideal solution and hope that an 8 ohm driver will be added to the Zu lineup
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